SMS Billing Confirmation Upon Saving - Output Type and Program/Form

Good day SAP World,

My company is busy with the process to send SMS confirmations to customers upon saving of the Sales Order and Billing Document. I have completed the spec for the Output of the Order but seems to hit a wall with the Invoice. Can anyone please guide me in which Output Type to copy with re to Billing Output and Program to use and would the medium be 7 or 5 ( I used 5 for the sales order)